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WATCH: U.S. President Barack Obama has authorized sending weapons to Syrian rebels after the White House disclosed that the United States has conclusive evidence that President Bashar al-Assad's government used chemical weapons against opposition forces. (AP)

Syria has rejected U.S. claims that the Syrian regime has used chemical weapons in its fight against rebels.

On June 14, Syria's Foreign Ministry saying Washington's statement was "full of lies" and "based on fabricated information."

The White House said on June 13 that U.S. investigators had determined the Syrian government has used chemical weapons and that the United States would now start providing military aid to Syrian rebels.

It said 100 to 150 people have been killed across Syria as a result of the chemical weapons attacks by President Bashar al-Assad's regime -- including sarin nerve-gas attacks.

The White House also said no reliable evidence was found to suggest that rebel forces have used chemical arms.

U.S. President Barack Obama has previously called the use of chemical weapons a "red line" that would lead to increased U.S. involvement in the Syrian conflict.

Until now, the United States has been providing "nonlethal" aid to the opposition.

It was not immediately clear what type of military aid Washington will provide to the Syrian opposition’s Supreme Military Council (SMC) in the future.

But speaking to reporters in a conference call, deputy national security adviser Ben Rhodes said Washington was ready to begin providing direct support to the opposition that would be "different in both scope and scale" to what the United States had previously provided.

"The president has made a decision about providing more support to the opposition. That will involve providing direct support to the SMC. That includes military support," Rhodes said.

"I cannot detail for you all of the types of that support, for a variety of reasons, but, again, suffice to say, this is going to be different in both scope and scale in terms of what are providing to the SMC than what we have provided before."

Russia 'Unconvinced'

Rhodes also said Washington had provided information to Moscow about the use of chemical weapons by Assad’s regime.

But Russian President Vladimir Putin's top foreign-policy adviser said on June 14 that the information the United States had given Russia about suspected use of chemical weapons by the Syrian government "does not look convincing."

Yury Ushakov said the U.S. decision to expand military support for Syrian rebels would undermine joint efforts by Moscow and Washington to organize a peace conference.

Aleksei Pushkov, chairman of the Russian State Duma's Foreign Affairs Committee, added: "The U.S. National Security Council, which made this statement about the use of chemical weapons, cites U.S. intelligence. The problem is that some time ago the Bush administration also cited U.S. intelligence in trying to prove to the whole world that Saddam Hussein had weapons of mass destruction."

Michael Mann, the spokesman for EU foreign-policy chief Catherine Ashton, said in Brussels on June 14 that Washington's findings show that there is an urgent need to get United Nations monitors into Syria as soon as possible.

"This assessment and others that have also been circulated makes it even more important that a UN verification mission be deployed to Syria to investigate these allegations on the ground," Mann said.

British Foreign Secretary William Hague said on June 14 that Britain agreed with the U.S. assessment, while NATO Secretary-General Anders Fogh Rasmussen said he welcomed the "clear" U.S. statement.

What Kind Of Arms?

The announcement on June 13 came with the Syrian rebels seeking more sophisticated weapons after a series of battlefield losses to regime forces, who have recently been bolstered by Lebanese Shi’ite Hizballah militants and Iranian fighters.

The U.S. announcement was quickly followed by calls from some U.S. lawmakers for Washington to provide antitank and surface-to-air missiles to the rebels. Syrian rebel and political opposition leaders also called for urgent deliveries of antiaircraft guns and other sophisticated weaponry.

Until now, Sunni-led U.S. allies Saudi Arabia, Qatar, and Turkey have reportedly supplied the rebels with arms.

During the conflict, Russia has continued to supply weapons to the Syrian government, a longtime ally of Moscow.

France and Britain earlier said they had found evidence that Assad’s forces had used chemical weapons. The two European Union powers recently succeeded in getting the EU to lift its embargo on providing weapons to the rebels.

The U.S. decision to begin supplying military aid follows a debate within the Obama administration on the wisdom of taking such a step. Some U.S. officials are concerned weapons could wind up in the possession of anti-Western militants such Al-Qaeda, who have been backing the rebellion.

Meanwhile, following a meeting on June 13, leading Sunni Muslim clerics called for a holy war against the Assad regime and its allies.

The clerics issued a statement saying: "The flagrant aggression of the Iranian regime, of [Lebanon’s] Hizballah, and of their sectarian allies in Syria amounts to a declaration of war against Islam and Muslims."

The Sunni clerics said "jihad" must be undertaken "to help our brothers in Syria by sending them money and weapons."

Those backing the call include Abdulaziz al-Shaikh, Saudi Arabia's top cleric; Youssef al-Qaradawi, a high-profile Egyptian preacher based in Qatar; and Hassan al-Shafai, a senior scholar from Egypt's leading religious academy Al-Azhar.

Lebanon's Iran-backed Shi'ite movement has been fighting alongside the forces of Assad, who is a member of the Alawite offshoot of Shi'a Islam.

The U.S. announcement was made after the United Nations raised its estimate of the death toll from the Syrian conflict, which is now in its third year, to nearly 93,000.